FIG. 3 is a perspective view of a ridge waveguide type visible-light laser diode having a double heterojunction structure in accordance with the prior art. In FIG. 3, reference numeral 1 designates an n type GaAs substrate. An n type AIGaInP lower cladding layer 2 is disposed on the substrate 1. A p type AlGaInP lightguide layer 4 is disposed on the active layer 3. A p type GaInP etch-stop layer 5 is disposed on the lightguide layer 4. A p type AlGaInP upper cladding layer 6 is disposed on the etch-stop layer 5. A p type GaAs cap layer 7 is disposed on the upper cladding layer 6. The upper cladding layer 6 and the cap layer 7 are formed by selective etching using the etch-stop layer 5, resulting in a stripe-shaped ridge 10 extending between the resonator facets 13 and 14. An n type GaAs current blocking layer 8 is disposed on the etch-stop layer 5 to bury the ridge 10. A p type GaAs contact layer 9 is disposed on the current blocking layer 8 and the ridge 10. An n side electrode 11 is disposed on the rear surface of the substrate 1 and a p side electrode 12 is disposed on the contact layer 9.
A method for fabricating the visible-light laser diode of FIG. 3 is illustrated in FIGS. 4(a) to 4(g). In these figures, the same reference numerals as those shown in FIG. 3 designate the same parts, and reference numeral 20 designates a silicon nitride (hereinafter referred to as SiN.sub.x) layer. Arsine (hereinafter referred to as AsH.sub.3) 21 and trimethylgallium (hereinafter referred to as TMG) 22 are used as the As source and the Ga source, respectively.
As shown in FIG. 4(a), a p type AlGaInP lower cladding layer 2, a GaInP active layer 3, a p type AlGaInP lightguide layer 4, a p type GaInP etch-stop layer 5, a p type AlGaInP upper cladding layer 6 and a p type GaAs cap layer 7 are successively grown on an n type GaAs substrate 1 (first crystal growth). Thereafter, as shown in FIG. 4(b), a SiN.sub.x film 20 is deposited on the cap layer 7 and, as shown in FIG. 4(c), it is patterned into a stripe configuration by photolithography and etching techniques. Then, as shown in FIG. 4(d), using the patterned SiN.sub.x film 20 as a mask, the p type AlGaInP upper cladding layer 6 and the p type GaAs cap layer 7 are partially etched away by selective etching using the etch-stop layer 5, resulting in a ridge 10. Then, using the SiN.sub.x film 20 on the ridge 10 as a mask for selective growth, an n type GaAs current blocking layer 8 is selectively grown on the etch-stop layer 5 to bury the ridge 10. Here, AsH.sub.3 is used as the As source and TMG is used as the Ga source. When TMG 22 is used as the Ga source, as shown in FIG. 4(e), the n type GaAs layer is not grown on the side walls of the ridge 10 comprising AlGaInP series material but grows as flat layer on the GaInP etch-stop layer 5, resulting in the n type GaAs current blocking layer 8 shown in FIG. 4(f) (second crystal growth). Then, as shown in FIG. 4(g), the SiN.sub.x film 20 is removed and a p type GaAs contact layer 9 is grown on the current blocking layer 8 and the ridge 10 (third crystal growth). Thereafter, a p side electrode 14 and an n side electrode II are formed on the contact layer 9 and the rear surface of substrate 1, respectively, by sputtering. Then, the wafer is divided into chips, resulting in the visible-light laser diode shown in FIG. 3
In accordance with the prior art method for fabricating a visible light laser diode, since TMG is used as the Ga source in the second crystal growth for growing the n type GaAs current blocking layer, the GaAs layer is not grown on the side walls of the ridge comprising AIGaInP series material. Therefore, during the second epitaxial growth, the side walls of the ridge are exposed to a high temperature in an AsH.sub.3 atmosphere for a long time and phosphorus escapes from the side walls of the ridge, whereby the double heterojunction structure is damaged, resulting in deterioration of the characteristics of the laser.
Referring to FIG. 5(a), when the GaAs current blocking layer is formed using triethylgallium (hereinafter referred to as TEG) as the Ga source, the GaAs layer 8' is grown on the side walls of the ridge 10, so that the side walls of the ridge are completely covered with the GaAs layer. However, since the growth of the GaAs layer 8' proceeds vertically from the surface of the etch-stop layer and horizontally from the side walls of the ridge as shown in FIG. 5(a), cavities 30 are undesirably produced in the GaAs current blocking layer 8' as shown in FIG. 5(b), adversely affecting the current blocking characteristic.